<template>
  <scroll class="listview" 
          :data="singers" 
          ref="listview" 
          :listenScroll="listenSrroll"
          @scroll="scroll"
          :probeType="probeType"
          >          
    <ul>
      <li class="list-group" v-for="(group, index) in singers" :key="index" ref="listGroup">
        <h2 class="list-group-title">{{group.title}}</h2>
        <ul>
          <li @click="selectItem(item)" v-for="(item, i) in group.items" :key="i" class="list-group-item">
            <img v-lazy="item.avatar" alt="" class="avatar">
            <span class="name">{{item.name}}</span>
          </li>
        </ul>
      </li>
    </ul>
    <div class="list-shortcut" @touchstart="onShortcutTouchStart" @touchmove.stop.prevent="onShortcutTouchMove">
      <ul>
        <li v-for="(item,index) in shortcutList" :key="index" class="item"  :data-index="index" :class="{'current':currentIndex === index}">
          {{item}}
        </li>
      </ul>
    </div>
    <div class="list-fixed" ref="fixed" v-show="fixedTitle">
      <div class="fixed-title">{{fixedTitle}}</div>
    </div>
    <div v-show="!singers.length" class="loading-container">
      <loading></loading>
    </div>
  </scroll>
</template>

<script>
import Scroll from 'base/scroll/scroll'
import Loading from 'base/loading/loading'
import {getData} from 'common/js/dom'
const ANCHOR_HEIGHT = 18 // shortcutList中 每个li的高度
const TITLE_HEIGHT = 30 //每一个标题的高度
export default {
  data() {
    return {
      scrollY: -1,//检测实时滚动的位置
      currentIndex: 0,
      probeType:3,//滚动监听
      diff:-1
    }
  },
  props: {
    singers: {
      type: Array
			}
  },
  components: {
    Scroll,Loading
  },
  created() {
    this.touch = {} // 用来传递点击开始与滑动的之间的变量
    this.listenSrroll = true
    this.listHeight = []
  },
  mounted() {},
  methods: {
		selectItem(item){
			this.$emit('select',item)
		},
    onShortcutTouchStart(e) {
      let anchorIndex = getData(e.target, 'index')
      let firstTouch = e.touches[0] // 记录按下的第一位置
      this.touch.y1 = firstTouch.pageY
      this.touch.anchorIndex = anchorIndex // 一开始点击的那个锚点索引值
      // console.log(1, firstTouch)
      this._scrollTo(anchorIndex)
    },
    onShortcutTouchMove(e) {
      let firstTouch = e.touches[0]
      this.touch.y2 = firstTouch.pageY // 记录滑动的位置
      //  (this.touch.y2 - this.touch.y1) / ANCHOR_HEIGHT 偏移了几个锚点
      let delta = (this.touch.y2 - this.touch.y1) / ANCHOR_HEIGHT | 0 // 偏移到的某个锚点值
      let anchorIndex = parseInt(this.touch.anchorIndex) + delta
      // console.log(anchorIndex)
      // console.log(2, firstTouch)
      this._scrollTo(anchorIndex) // 移动的时候获取到滚动的第几个位置
    },
    scroll(pos){
      this.scrollY = pos.y; //获取竖向滑动的距离
      // console.log(pos)
    },
    _scrollTo(index) {
      console.log(index) //滑动到最顶部的时候,此时index为负值,故作如下判断处理index边界问题
      if(!index && index !== 0){
        return 
      }
      if(index < 0){
        index = 0
      }else if(index > this.listHeight.length - 2){
        index = this.listHeight.length - 2

      }
      this.scrollY = -this.listHeight[index] //上限的位置
      this.$refs.listview.scrollToElement(this.$refs.listGroup[index], 0)
    },
    _calculateHeight() { //计算左侧每一个list的高度
      this.listHeight = []
      const list = this.$refs.listGroup
      let height = 0
      this.listHeight.push(height)
      for(let i=0;i< list.length;i++){
        let item = list[i]
        height += item.clientHeight
        this.listHeight.push(height)
      }
      // console.log(this.listHeight.length)  // 作为右边联动的一个区间值集合
    } 
  },
  computed: {
    shortcutList() {
      return this.singers.map(group => {
        return group.title.substr(0, 1)
      })
    },
    fixedTitle(){
      if(this.scrollY > 0) {
        return ''
      }
      return this.singers[this.currentIndex] ? this.singers[this.currentIndex].title:''
    }
  },
  watch: {
    singers() { //dom与与数据加载之间有延时,故此处作如下处理 
      setTimeout(()=>{
        this._calculateHeight(); 
      },20)
    },
    scrollY(newY){
      let listHeihgt = this.listHeight

      //1.滚动到顶部,newY > 0
      if(newY > 0) {
        this.currentIndex = 0
        return
      }

      //2.中间部分滚动
      for(var i=0;i<listHeihgt.length;i++){
        let height1 = listHeihgt[i] //上限
        let height2 = listHeihgt[i+1]//下限
        if(!height2 || (-newY>=height1 && -newY<height2)){  //滚动区每一个group所在的区间
          this.currentIndex = i
          this.diff = height2+newY //当前list区块与fixedTitle之间的距离 滚动以后两个title之间距离
          // console.log(this.currentIndex)
          return
        }
      }

      //3.当滚动到底部,且-newY大于最后一个元素的上限
      this.currentIndex = listHeihgt.length - 2
      // console.log(this.currentIndex)
    },
    diff(newVal){
      let fixedTop = (newVal > 0 && newVal < TITLE_HEIGHT) ? newVal - TITLE_HEIGHT : 0
      if(this.fixedTop === fixedTop){
        return
      }
      this.fixedTop = fixedTop
      //transform 减少dom操作的频度,达到优化的效果
      this.$refs.fixed.style.transform = `translate3d(0, ${fixedTop}px,0)` //开启GPU加速
    }
  }
}
</script>

<style scoped lang='stylus'>
@import '~common/stylus/variable'
.listview
  position: relative
  width: 100%
  height: 100%
  overflow: hidden
  background: $color-background
  .list-group
    padding-bottom: 30px
    .list-group-title
      height: 30px
      line-height: 30px
      padding-left: 20px
      font-size: $font-size-small
      color: $color-text-l
      background: $color-highlight-background
    .list-group-item
      display: flex
      align-items: center
      padding: 20px 0 0 30px
      .avatar
        width: 50px
        height: 50px
        border-radius: 50%
      .name
        margin-left: 20px
        color: $color-text-l
        font-size: $font-size-medium
  .list-shortcut
    position: absolute
    z-index: 30
    right:0
    top: 50%
    transform: translateY(-50%)
    width: 20px
    padding: 20px 0
    border-radius: 10px
    text-align: center
    background: rgba(0, 0, 0, 0.3)
    font-family: Helvetica
    .item
      padding: 3px
      line-height:1
      color: rgba(255, 255, 255, .5)
      font-size: 12px
      &.current
        color:$color-theme
  .list-fixed
      position: absolute
      top: 0
      left: 0
      width: 100%
      .fixed-title
        height: 30px
        line-height: 30px
        padding-left: 20px
        font-size: $font-size-small
        color: $color-text-l
        background: $color-highlight-background
  .loading-container
      position: absolute
      width: 100%
      top: 50%
      transform: translateY(-50%)         
</style>